This invention relates to a ladder assembly mounted on large mobile vehicles, and more particularly, relates to retractable ladder assemblies for allowing access to an operator station or maintenance location on large mobile vehicles, such as large earth-moving vehicles.
While certain embodiments of the present invention are disclosed herein for use on certain types of mining and/or earth-moving vehicles, such disclosure and embodiments are by way of example and for clarity of understanding. As noted herein, the present invention has application beyond the uses explicitly disclosed herein.
Large mobile vehicles that are used to haul earthen materials for road construction or for mining are typically very large, powered vehicles. Such vehicles have very high ground clearances, which require some sort of ladder for the operator to be able to make the transition from ground level to the operator station or cab which is normally atop the powered vehicle. The prior art ladders are typically fixed ladders that start at about the bumper level and extend up to a walkway and handrail which then, at least partially, surround the operator station or cab area.
As the size and complexity of large mining and/or earth-moving vehicles increase, providing easy access to the operator station (or operating cab) and/or other maintenance or service areas is a recurring problem. The use of permanently fixed ladders on such vehicles necessarily within easy reach of the ground has not been completely satisfactory. This is especially true, since typically, the bottom portions thereof are usually placed well below the normal ground clearance of the vehicle and are therefore in a position to be damaged during normal vehicle operations. In prior art attempts to solve this problem various types of retractable steps or ladders have been devised that may be lifted or pivoted out of the way (U.S. Pat. Nos. 3,603,429 and 3,826,337). However, most of these prior art ladders are still subject to being damaged and in some cases can be shaken or jarred loose during routine vehicle operations. These ladders also require operator effort to return them to their stored position, something that is not always done.
Other prior art attempts to solve this access problem have employed some type of lower steps to make the transition to such a fixed ladder, and are usually a flexible step (or steps) that are supported by a wire rope, chain, or rubber. This step (or steps) is suspended from the bottom of either a bumper or some other appropriate frame or body element. However, such steps are generally still high above the ground and/or fixedly attached high above the ground (to a bumper), which causes an excessive amount of deflection of such a lower step away from the operator on his initial "step" onto such a lower step and makes the mounting or dismounting of the vehicle extremely difficult and hazardous.
Typical accidents that occur are the result of slips and falls from wet or icy steps while mounting and dismounting such large mobile vehicles. As is apparent in wet, muddy conditions the operator's boots or other footware become covered with materials, which often act as a lubricant and also coat the rungs/steps of any ladder. Such materials may also interfere with adequate gripping of the rungs/step by the operator's hands, whether gloved or not. Such slipping and falling type accidents are normally minor in severity, but they do result in substantial amounts of employee lost time and lost productivity. These injuries would characteristically include lacerations, contusions, fractures, and sprains.
In 1983, the U.S. Bureau of Mines sponsored a research program for the development of an improved truck ladder by Woodward Associates. A major result of that program was the development of a pretension spring ladder disposed below a bumper or appropriate frame element. Because of the fixed nature of the spring ladder and its suspension below a bumper it has not been entirely satisfactory. That is, it suffered the same shortcomings as the other prior art ladders using wire rope, chain, or rubber to suspend steps below a bumper (or other frame element). Further, the other prior art approaches to solving the problem have included folding steps, cable steps, rubber steps, and in some instances no bottom step at all. All these attempts have not been entirely satisfactory.
To solve this problem, self-retracting ladders have been proposed. However, these are typically limited to a single step and are thus not suitable for applications where substantial ground clearance of a vehicle is involved (U.S. Pat. No. 3,584,704). Other prior art ladders include a plurality of steps and a pantograph link arrangement with biasing springs to return the ladder to the stored position (U.S. Pat. No. 3,012,633); this ladder is not entirely satisfactory since it is the same size in its stored and operative positions. A self-retracting telescoping ladder (U.S. Pat. No. 3,869,022) is also a prior art ladder not entirely satisfactory in operation. Other prior art solutions are found in U.S. Pat. Nos. 4,217,971, 3,951,431, 4,021,071, 4,131,293, 4,161,997, 4,245,716, and 4,153,138 (hinge and stabilizer). These solutions are also not entirely satisfactory in operation.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and improved apparatus and methods are provided for easy and safe access to the operator station or maintenance locations of large mobile vehicles, such as large mining and/or earth moving vehicles.